Ideally, the water in a boiler feedwater/steam condensate return system is free of both dissolved and particulate matter. Even low levels of certain contaminants, if present in the condensate, can be detrimental to plant performance, resulting in reduced efficiency, production loss and plant shutdowns.
Unlike raw water, steam condensate is introduced into boiler feedwater/steam condensate return systems at temperatures well above ambient. Recognizing that substantial quantities of energy can be saved by recycling hot condensate, plant engineers have sought, for processes utilizing steam, to maximize the heat content and quantity of condensate returned to the boiler. Increasingly, these engineers are using condensate formed during startup and as production rates are adjusted, even though such activities can result in surges in the concentrations of impurities present in the condensate. If sent to a boiler, these surges can lead to increased fouling and higher rates of boiler tube failure.
One of the most undesirable contaminants in condensate is iron oxide. Not only is iron oxide difficult to treat chemically once it enters the boiler but also this substance has a great affinity for forming deposits within boiler tubes in high heat flux areas. Such deposits of iron oxide cannot easily be removed on-line. Generally, acid must be used to remove these deposits and to insure continued boiler reliability.
Produced by the natural forces of erosion and of corrosion acting within each steam/condensate system, iron oxide is inherently formed with condensate. Attempts, whether using mechanical or chemical means, to minimize the formation of iron oxide are at best imperfect. In addition to startups and shutdowns of plant processes, situations which can contribute to iron oxide being present in unacceptably high concentrations are: an inadequate or poorly controlled condensate treatment program; cross-contamination of condensate with process chemicals; and malfunctioning of equipment employed to pretreat boiler feedwater.
In most plants, normal operating practice has been to send contaminated condensate to the sewer. Unfortunately, deciding when one should open dump valves to release contaminated condensate has been fraught with uncertainty. Plant operators have not had a simple and reliable method at their disposal for continuously monitoring condensate contaminants and iron oxide in particular. Unfortunately, traditional condensate monitoring techniques, which rely on conductivity and pH measurements, do not provide an accurate indication of the concentrations of insoluble impurities such as iron oxide. Insoluble hardness levels, for example, can be excessive even though soluble hardness levels are below detection limits.
As for measuring iron oxide concentrations, many plant operators still rely upon a grab sample method of analysis which was developed about 30 years ago by the Babcock Wilcox Company. In this analysis, one basses a 1-liter sample through a 0.45 micron membrane filter and then compares the appearance (color, darkness and the like) of the filter bad with known "standards". Since the appearance of the filter bad can vary with, among other things, when and how the operator takes the sample and the analyst's interpretation of the experimental results, plant operators, depending upon such analyses, can easily find themselves underfeeding or overfeeding corrosion control chemical(s).
While more precise measurements based on chemical analysis of iron oxide concentrations can be performed, such measurements cannot, as a practical matter, be performed on-line. Photometric methods, for example, are encumbered by the fact that time-consuming acid digestion is required to convert ferric iron to soluble ferrous iron. Similarly, precision measurements based on atomic adsorption necessitate costly time delays. These measurements must be obtained using extremely expensive test equipment. Because such equipment is not suitable for most plant environments, samples usually must be transported off-site. Several days may pass before results are known, making the data useless to operators who must make decisions quickly to protect a system.
To provide an on-line analysis of condensate quality, some plants monitor for particulate matter, but not for iron oxide concentration. This approach is of value because, ordinarily, the level of particulate matter is indicative of the level of iron oxide, a major component of this particulate matter. In the past, turbidimeters have been used to obtain surrogate measurements for particulate iron in condensate.
Turbidimeters, however, measure turbidity rather than the actual numbers or concentration of particles. Turbidity, an expression of the optical property that causes light to be scattered and absorbed rather than transmitted in straight lines through an aqueous sample, is affected by the size, shape and composition of particles in the sample. Importantly, for a sample of unit volume having a given mass of particles, the turbidity is less when the particles are larger in size. As a consequence, relatively large particles can be present in a sample but not contribute much to its measured turbidity.
Moreover, several conditions affecting the optical surfaces of the walls of the sample cell can interfere with the accuracy of a turbidimeter. These surfaces may be imperfect due to the presence of dust, scratches, smudges, condensate or, especially in the case of continuous sampling units, an accumulated film formed thereon. While film formation can be abated with regular cleaning of the cell, fogging of the cell walls is a persistent source of interference when flows at elevated temperatures, such as steam condensate flows, are monitored.
Instruments known as particle counters, which are capable of sizing as well as counting particles, can be employed to overcome some of the limitations of turbidimeters. Accurate particle counting, however, is limited to those situations in which the concentrations of particles are low enough to avoid coincidence effects caused by the simultaneous passage of two or more particles through the sensing zone of the particle counter.
In practice, particle counters utilize narrow, gate-like sieves in a technique known as hydrodynamic focusing to minimize coincidence effects. Typically, these sieves allow only particles which are about 65 microns or smaller in size to pass through them. As long as all of the particles are small enough to pass through the sieve upstream of each particle counter in a system, plugging is prevented. When a flow blockage occurs, however, the instrument gives an erroneously low particulate matter concentration. This error arises because in particle counting, it is automatically assumed that the flow rate through the sensing zone of the counter remains constant and that the output of the counter is inversely proportional to this fixed flow rate even when the flow rate drops precipitously.
Partly as a result of particle size limitations, particle counters have not found utility in the steam condensate field but rather are used primarily for monitoring filtered water in the potable water treatment field. Unlike condensate which may contain large rust particles, filtered water has mainly small particles and very few or no particles greater than 20 microns in size.
Even in the potable water field, acceptance of particle counters has not been widespread. Utilities have been slow to adopt them because of their cost, operators' lack of familiarity with the instrument, and the tremendous, even excessive, amount of detailed information which particle counters generate. Turbidity measurement has remained the standard method of determining the efficiency of clarification and filtration processes in removing particles from raw water.
An alternative method developed for the on-line monitoring of concentrations of particles in water involves the use of an apparatus described by Gregory add Nelson in U.K. Patent GB 2,129,549B known as a particle monitor. The sensitivity of the particle monitor, like that of the particle counter, is dependent upon the size of particles being measured. For very small, colloidal particles, turbidimeters give better detection than particle monitors. For particles larger than 2.0 microns, the particle monitor has a detection capability 100 times greater than that of a turbidimeter and can be used to detect sub-ppb of such particles.
Instead of providing, as does the particle counter, counts of particles within several size ranges, the particle monitor has an output which is limited to a single number known as a particle index. This difference is due to the fact that the particle monitor utilizes an averaging technique in which a ratio is measured between fluctuations in the instantaneous intensity and the average intensity of a light or infrared radiation beam transmitted through a suspension flowing through a transparent plastic tube. As a particle passes between the light source and the detector, a shadow is created, causing a fluctuation. Importantly, measurement of these fluctuations eliminates the need for hydrodynamic focusing or a like technique to avoid coincidence effects.